1. Field of the Invention
This invention relates to a lithium battery.
2. Description of the Related Art
Accompanying wit the recent development of portable electronic equipments, batteries of higher performance have been demanded. Lithium ion batteries using a carbonaceous material in the negative electrode and lithium cobaltate (LiCoO.sub.2), which is a complex oxide having a layer structure, in the positive electrode have been put to practical use as a nonaqueous battery having a high working voltage and a high energy density. Because resources for lithium cobaltate are scarce and expensive, a lithium-containing manganese oxide or lithium nickelate has been proposed as a substitute therefor. A lithium-containing manganese oxide has a low theoretical capacity density, and the capacity decreases with charge and discharge cycles.
On the other hand, lithium nickelate (Li-containing nickel oxide; LiNiO.sub.2) is a compound having the same layered crystal structure as lithium cobaltate, having lithium ions intercalated between layers of edge-sharing NiO.sub.6 octahedra. Lithium nickelate is generally prepared by mixing a nickel source such as Ni(NO.sub.3).sub.2, Ni(OH).sub.2, NiCO.sub.3, NiO, and NiOOH, and a lithium source such as LiOH, LiNO.sub.3, Li.sub.2 CO.sub.3, and Li.sub.2 O.sub.2, and the mixture is heated at about 600 to 900.degree. C. in an oxygen stream.
However, as reported in Solid State Ionics, Vol. 44, 87 (1990), Chem. Express, 7, 689 (1992) or Dai 33-kai Denchi Toronkai Koen Yoshishu, P. 21 (1992), the structure of lithium nickelate is similar to a rock salt structure so that nickel and a lithium ion are easily exchanged to produce a disordered structure, causing a degradation in capacity.
Use of nickel oxyhydcroxide as a nickel raw material has been attempted. For example, JP-A-63-19760 (the termn "JP-A" as used herein means an "Unexamined Japanese Patent Publication" (kokai)) proposes using nickel oxyhydroxide containing a cobalt of 20 to 75% as an active material for lithium batteries, but the discharge characteristics are not so satisfactory. In order to improve the discharge characteristics, JP-A-6-31045 proposes mixing a hydroxide or oxide containing a trivalent nickel with a lithium salt and heating the mixture. According to the publication, a sodium hydroxide solution having dispersed therein divalent nickel hydroxide (Ni(OH).sub.2) is reacted with an aqueous solution of sodium hypochlorite to prepare nickel oxyhydroxide, and a hydroxide or oxide containing the nickel oxyhydroxide is mixed with lithium nitrate. The mixture is pelletized, dried and heated in air at 600 to 800.degree. C. The mixture is once ground, again pelletized, and heated in air at 700 to 900.degree. C. to prepare lithium nickelate.
These processes utilizing nickel oxyhydroxide encounter difficulty in preparing pure lithium nickelate, and, above all, the resulting lithium batteries have a serious disadvantage that the voltage during charging/discharging process varies in many stages, e.g., 4 stages, and the performance at a higher discharging current also:reduces. Besides, nickel oxyhydroxide must be synthesized as a starting material in order to synthesize lithium nickolate by these processes, which makes the production complicated and incurs cost.
As stated above, lithium nickelate in generally synthesized by mixing a nickel compound and a lithium compound and burning the mixture at a high temperature. However, batteries containing lithium nickelate obtained in this way as a positive active material are unsatisfactory in charge/discharge characteristics and capacity. Further, this process requires equipment enabling high temperature processing. Furthermore, the production steps are complicated and costly depending on starting materials.
Accordingly, there has been a demand for a battery containing a novel positive active material which can be synthesized through a simplified process that can be carried out in low temperature and at low cost Unlike the conventional high temperature processing.